A Single Catalyst for Promoting Reverse Processes: Synthesis and Chemical Degradation of Polylactide

Abstract A simple zinc catalyst showing high activity for both the synthesis of polylactide, a biodegradable polymer produced from renewable feedstock, and its degradation was described. In the ring‐opening polymerization of lactides, the zinc catalyst showed one of the highest activities reported in the literature for reactions carried out in solution at room temperature. This excellent performance was preserved even when the process was performed under industrial conditions: at high temperature, in the absence of solvent, and by using a low catalyst loading with unpurified monomers. The same complex revealed high efficiency also in depolymerization of polylactide by alcoholysis, a process that occurred efficiently at room temperature and in the absence of solvent, conditions that reduce costs and guarantee low environmental impact.

The success of performed reaction was confirmed by 1 H NMR experiment, which spectrum is illustrated in the following figure: Figure S1. 1 H NMR spectrum (400 MHz, C 6 D 6 , 298 K) of ligand L1.

Synthesis and characterization of the complex 1
Scheme2. Synthesis of complex 1.
To a stirred solution containing Zinc bis[bis(trimethylsilyl)amide] (773 mg, 2.0 mmol) in dry benzene (4.0mL) was added dropwise a solution of the ligand precursor L1 (453 mg, 2.0 mmol) in dry benzene (2.0mL). The solution was stirred for 3h at room temperature. The solvent was removed under vacuum, forming a red solid. Yield: 80%. The formation of the desired species was confirmed by NMR and elemental analysis. Elemental Analysis calculated for: ,53.26;H,6.93;N,9.32;Found: C,52.98;H,7.02;N,9.47.

General procedure for the polymerization of lactide at room temperature
The polymerization was carried out under inert conditions. In a Braun Labmaster glovebox, a magnetically stirred reactor vessel (10 mL) was charged with lactide. The metal-complex and iPrOH (0.1 M solution of iPrOH in dichloromethane) were added in a 4mL vial and mixture was stirred for 5' then added dichloromethane and transferred in the monomer. The reaction mixture was stirred at room temperature. During the reaction, small aliquots of the reaction mixture were sampled, dissolved in CDCl 3 and analyzed by 1 H NMR spectroscopy. At the end of the polymerization the product was precipitated in hexane then filtered and dried in a vacuum oven.

General procedure for the polymerization of lactide at high temperature
The polymerization was carried out under inert conditions. In a Braun Labmaster glovebox, a magnetically stirred reactor flask (50 mL) was charged with lactide. The metal-complex and 1 equivalent of BzOH (0.1 M solution of BzOH in toluene) were added in a 2 mL vial and the mixture was stirred for 5' then transferred in the monomer and, finally, mixed to more equivalents of BzOH. The reaction mixture was stirred at 150°C. During the reaction, small aliquots of the reaction mixture were sampled, dissolved in CDCl 3 and analyzed by 1 H NMR spectroscopy. At the end of the polymerization the product was precipitated in hexane then filtered and dried in a vacuum oven.    Table S1. Figure S16. GPC analysis of sample 12 Table 1.    Figure S19. DSC measurement of a P(L-LA) sample run 10 Table 1. Table S1. Polymerization of lactides promoted by 1. a All reactions were carried out by using 10 µmol of Zn catalyst at 25 °C in CH 2 Cl 2 (2 mL), ROH was i PrOH for runs at 25 °C and BzOH for runs at 150 °C. b Experimental M n (corrected using factor of 0.58) and Ð values were determined by GPC analysis in THF using polystyrene standards. c Calculated M th (KDa) =144.14 x ([LA]/[ROH]) x conversion of LA. d LA purified by a single crystallization e unpurified LA. f technical grade purity LA, solvent free. e Reaction performed in THF.

General procedure for the depolymerization of polylactide in solution
The depolymerization reaction was carried out under an inert atmosphere. In a Braun Labmaster glovebox, a magnetically stirred reactor vessel (10 mL) was charged with polylactide and MeOH. In a vial (4mL) the metal-complex was dissolved in THF and added to the polymer. The reaction mixture was stirred at room temperature. At desired times, small aliquots of the reaction mixture were sampled, dissolved in CDCl 3 or CD 2 Cl 2 and analyzed by 1H NMR spectroscopy. At the end of the depolymerization the reaction was stopped with CH 2 Cl 2 and dried under vacuum.

General procedure for the depolymerization of polylactide solvent free
The depolymerization reaction was carried out under an inert atmosphere. In a Braun Labmaster glovebox, a magnetically stirred reactor vessel (10 mL) was charged with polylactide. In a vial (4mL) the metal-complex was dissolved in MeOH or EtOH and added to the polymer. The reaction mixture was stirred at room temperature. At desired times, small aliquots of the reaction mixture were sampled, dissolved in CDCl 3 or C 6 D 6 and analyzed by 1 H NMR spectroscopy. At the end of the depolymerization the reaction was stopped with CH 2 Cl 2 and dried under vacuum.    All reactions were carried out by using 5 µmol of Zn catalyst (0.6 mol % relative to ester linkages) in 1.8 mL of THF, with 0.2 mL of MeOH b M n = the experimental (GPC) values of the recorded molecular weight of the polymer. c Determinated by 1 H NMR; d reaction performed in air.